Can someone explain high pressure side and low pressure side? I understand vacuum is it the high pressure side you vacuum?
Can someone explain high pressure side and low pressure side? I understand vacuum is it the high pressure side you vacuum?
In reply to TRoglodyte :
You want the whole system Under vacuum
So you pull a vacuum on which side?
The AC system is a loop. It's divided into two 'sides', a high (pressure) side and a low (pressure) side. The dividing lines are the compressor and the metering device. A metering device is a restriction in the system. Some are fixed orifices (orifice tube) and some are self-adjusting based on temperature just like a thermostat (thermal expansion valves, or TXVs).
The orifices are never totally closed (unless broken). Because of this, no matter which side of the system you are hooked to with your vacuum pump, you are pulling the whole system under that vacuum. At the very low flows created by the vacuum pump, the metering device basically doesnt restrict at all. It's only when the compressor is running and creating a large flow that the metering device creates the pressure difference between the high and low sides. When the compressor is not running, pressure is the same everywhere in the loop. This is called static pressure.
It's also not a bad idea to replace your receiver/drier before repair and recharge since it's cheap and likely needs replacing with a system that is bottoming out on a leak.
I'd say go ahead and try it. There's a learning curve, for sure but it's not difficult to figure out.
For me the most difficult part has always been pulling dashboards to replace evap cores or expansion valves. A factory service manual with detailed procedures will save you HOURS when pulling one.
The little venturi pump from HF does work, but to pull 30" like Cousin Eddie mentioned it takes some serious airflow at about 120 psi.
They are complete garbage though. The venturi itself is made from plastic and is very brittle. I went through having to exchange several of them before a buddy gave me a Mac out of pity. LOL
The advantage in having all the tools is you can keep charging the system until it leaks bad enough to find leaks visually. 
With SWMBO'S Dakota the evaporator core eventually blew out bad enough you could smell the refrigerant coming from the vents. LOL
Because I scored an Amazon gift card for filling out some BS survey at work (Don't tell them I don't actually work in procurement!) and because I know how handy being equipped to do this sort of work can be, I went ahead and ordered up a vacuum pump and gauge set from Amazon plus some R134a cans and a can tap. Y'all are making this sound fairly simple, so I guess I don't really have anything to lose, right? I mean, worst case, my non-functioning A/C will remain non-functioning! I won't have time to do anything with it for a week or so, but I'll report back what I'm able to accomplish. Or maybe there will be some lightly-used, gently-thrown A/C repair tools for sale. Only time will tell!
I do need to order up the receiver/drier for each system, though. They're cheap enough, which is good, but they apparently used about fifteen different ones on the P30 chassis in 1993, so I've got to go put eyeballs on the one on the motorhome.
After you have replaced everything and vacuumed it out, do you add oil again with the “Freon?” I have heard both that you should and that it’s not necessary because the oil is probably still in the compressor.
Unfortunately, the amount of oil in the system is a grey area unless you are either flushing or replacing every part of the system. The refrigerant only carries the oil when moving quickly as it does when the system is running. When evacuating the system into a 'real ac machine' the flow is slower and only some of the oil will be carried out by the refrigerant. AC machines have an oil drain bottle that is marked in ounces, so you can simply add the amount of oil that you drained out back into the system. So, in the case of using an ac machine, this is less of an issue. Vacuum pumps themselves do not really remove oil at all. The purpose of the vacuum is to cause water to evaporate at <212f by lowering the pressure on it until its boiling point drops below ambient temp, but this doesn't work on oil. The oil will simply sit in there, for the most part. If you 'vent' a charged system to atmosphere quickly, you will see a visible fog coming out. The fog is the oil being picked up and carried by the fast moving refrigerant. If you vent it slowly, you will see nothing because the refrigerant is an invisible gas and isn't moving quickly enough to pick up the (visible) oil.
Some of the components are essentially impossible to get all the oil out of without flushing. Even then, something like an accumulator can't be emptied of oil even by flushing, although since that part also contains the dessicant in the system, it is usually replaced when the system is opened anyway.
As a tech i can look up total system oil capacity in AllData or similar. If a system is bone dry it will usually take something like 8 ounces, but this varies wildly based on age and system size, etc.
If i'm servicing a system without flushing or replacing any of it, i use a whole bunch of context clues to inform my judgment call about how much to add. What kind of leak led to me needing to add refrigerant, a small leak that didn't leak oil, or a fast leak that left a soaked hose/connection/etc? Has the owner been repeatedly 'topping off' the system without adding oil before having me fully repair it? Luckily, I think i can say that it's sort of like engine oil. Running low will just shorten lifespan but won't downright kill the compressor until there's almost nothing left. Having too much won't hurt much until it's WAY too much. So, it is possible to be 'off' a decent bit and still end up with a working, long-lived system.
You might want to get a copy of "Just Needs a Recharge" by Rob Siegel. I haven't read it yet but when I read his stuff I learn things.
In reply to Vigo :
That’s a really excellent explanation
In reply to Vigo :
Thanks, Vigo!
I went DIY years ago. Good gauges and a good vac pump. Add a heaping tablespoon of knowledge and you're good to go.
I have just replaced the entire system on the 2002 CRV due to a (common) compressor failure, and this is relevant. My manifold/gauges arrive tomorrow, so we'll see how this all goes!
Well, the vacuum pump, can tap, R134a, and gauges have all shown up. Tonight is Bingo, tomorrow night we're having company, but on Saturday I think I'm going to start on the motorhome's system. I need to get a new spray bottle and some cheap car wash to use to make bubbles, an adapter to fit the refrigerant hose to the can tap, and an adapter to be able to plug the air compressor into the refrigerant hose. I know the system will need a new drier, and those appear to be readily available at the FLAPS in the area, but otherwise I'm not sure what's up with the system, so it'll be an adventure!
Remember that joke I told about the drier being readily available? Ha. Ha.
Apparently, Winnebago couldn't just use the dash AC that Chevy put on their other p30s. No. They had to come up with their own system. Huzzah! So the drier that I have doesn't seem to match anything. Ordinarily, I suppose that wouldn't be a big problem: just get something, make a bracket, adapt the fittings, and done. This drier, however, has some sort of switch on it. In addition to the regular pressure switch that goes in the line. So I've special ordered the part that appears to work (3 FLAPS and about 90 minutes of searching catalogs) but it won't be in until next Friday.
To find leaks, I used a spray bottle with a 50/50 mix of car wash and water. I tested it on the air compressor fitting that I know leaks and instant major huge bubbles. After pressurizing the system with air, I sprayed everywhere and didn't get a single bubble. When I disconnected the high-side line from the gauge, the service port just started venting. At some point someone did a r134a conversion on the system and the adapter on the high side just blew out - no seal, no valve. Easy fix. If I could get a freaking drier...
I completed mine over the weekend, and it really wasn't all that bad! The CRV pulled ~28"Hg and held it, so I filled it up and now have COLD air whenever. The tricky bit is once the refrigerant cans get cold, they tend to slow down on filling the system, so it was fill-pause with the can behind the radiator fan-fill-repeat until the correct amount of refrigerant had been charged, then double checking pressures/temps according to the FSM.
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